Colorectal cancer (CRC) is one of the leading causes of cancer-related deaths in the Veterans armed forces personnel and each year Veterans affairs manages and treats ~175,000 colorectal cancer patients [1]. Although, significant advances are made in the screening/diagnosis of the disease, treatment options remain only moderately successful and primarily include surgical resection and chemotherapy. Importantly, disease progression and metastasis remain the prime cause of the morbidity and mortality among VA-CRC patients. Thus, new therapies with the promise to inhibit CRC progression are urgently needed. Our studies are an important step in that direction. Notably, extensive work from our and other laboratories has confirmed that expression of claudin-1 is highly upregulated in CRC in a stage-specific manner, and is causally associated with CRC growth and progression. Now, using a novel mouse model of intestine-specific claudin-1 overexpression, we show novel role of claudin-1 in the regulation of Notch-signaling and colonocyte maturation/goblet cell differentiation/Muc-2 expression. We further demonstrate multi-fold increase in colonic tumor burden (tumor incidence and growth) and aggressiveness in the offspring generated through the cross between APCmin and claudin-1 transgenic mice. In our continued study, we have now generated evidence that Notch- and Wnt-signaling synergize in claudin-1-dependent manner to augment CRC in APCmin/Cl-1 mice, possibly through the regulation of cancer stem cells. Taken together, our hypothesis is that increased claudin-1 expression induces Notch-hyper activation to dysregulate normal colonic differentiation and homeostasis, and thus induces susceptibility to CRC growth & progression through the synergy between Notch and Wnt- signaling. To test our hypothesis, we propose following specific aims: Specific Aim 1. Determine that claudin-1 expression modulates Notch-signaling to regulate colon tumorigenesis. Here we will: a) determine that Notch-signaling regulates increased tumorigenesis in APCmin/Cl-1 mice; b) determine whether mucosal inflammation contributes to Claudin-1-dependent increase in colon tumorigenesis and/or aggressiveness; c) determine the mechanism/s underlying Claudin-1-dependent regulation of Notch-activation. Specific Aim 2. To determine that Notch synergizes with Wnt/b-catenin signaling to regulate claudin-1- dependent increase in colon tumorigenesis. Here, we will determine: a) the impact of inhibition of Wnt/b- catenin-signaling on colon tumorigenesis in APCmin/Cl-1 mice.; b) that claudin-1-dependent increase in Wnt/b- catenin signaling depends upon Notch-activation and to examine the underlying mechanism/s; c) whether claudin-1 expression correlates with Notch and b-catenin/Wnt-activation in colon cancer patients and its potential association with the aggressiveness of cancer. Specific Aim 3. To investigate the role of APC/ Wnt-signaling in the regulation of colonic Claudin-1 expression. Here, we will: a) examine the role of APC (WT)-Smad4 axis in the regulation of Claudin-1 expression; b) determine the details of the cross-talk between APC and Smad4 in transcriptional regulation of Claudin-1. Our short term goal is to better understand how dysregulation of claudin-1 expression modulates the ability of colon cancer cells to form tumor and further progression. Our long term goal is to develop claudin-1-specific inhibitors that can be delivered specifically to the colon cancer cells and thus to create an anti-CRC drug that is not toxic and inhibits disease progression. We believe such therapeutic interventions can significantly increase survival and quality of life US Veterans who are suffering from colorectal cancer.